Other worlds
How more than five thousand planets around other stars were found without ever seeing one, what the habitable zone does and does not mean, and an honest look at the question of life.
METHOD ONE
Watch for a tiny, repeating dip
If a planet's orbit happens to lie edge-on from our direction, the planet passes in front of its star once every orbit and blocks a sliver of its light. The star does not vanish; it dims by a fraction of a percent, for a few hours, and then does it again at the same interval. That is a transit. A telescope that watches a hundred thousand stars at once and looks for repeating dips can find thousands of planets.
The catch is geometry, the same problem as the pulsar beams in lesson 07. Only a small fraction of planetary systems are lined up edge-on to us. Every transit found stands for many more planets whose orbits are tilted, which we miss entirely.
how much light does a transiting planet block?
it is the ratio of the two discs: (planet width / star width) squared
Jupiter across the Sun:
140,000 / 1,400,000 = 0.1 -> 0.1 x 0.1 = 0.01
Earth across the Sun:
12,700 / 1,400,000 = 0.009 -> 0.009 x 0.009 = 0.00008Jupiter dims the Sun by about 1%. The Earth dims it by about 0.008%.Eight parts in a hundred thousand, repeating on a fixed schedule, measured across light-years. That is the level of precision the transit method needs to find a planet like ours, and space telescopes have reached it.
Check yourself
Kaveh watches a star for two years and records three dips of 1%, each lasting five hours, spaced exactly 200 days apart. What is the most reasonable reading?
- A planet the same size as the star
- A star that pulses in brightness by itself
- A planet roughly a tenth of the star's width, orbiting every 200 days
- Clouds in the Earth's atmosphere passing over the telescope
Show the answer
A planet roughly a tenth of the star's width, orbiting every 200 days
Right. Even spacing means an orbit, and a 1% dip means the planet covers 1% of the star's disc, so about a tenth of its width. It is a Jupiter-sized planet with a 200-day year.
METHOD TWO
Watch the star wobble
A planet does not orbit a fixed star. The two swing around their common balance point, so the star traces a small circle of its own. When the star moves toward us its spectral lines shift blue, and when it moves away they shift red, over and over with the planet's year. This is the Doppler shift from lesson 02, used on a much smaller scale, and it is how the first planet around a Sun-like star was found in 1995.
The numbers are brutal. Jupiter makes the Sun move at about 12 metres per second, slower than a car in town. The Earth makes it move at about 9 centimetres per second, a slow walk, measured on a star trillions of kilometres away.
Why astronomers want both methods
A transit tells you SIZE
The depth of the dip gives the planet's width compared with the star's, and the spacing gives the length of its year. It says nothing about how heavy the planet is: a puffy gas ball and a dense rock of the same width block the same amount of light.
A wobble tells you MASS
How fast the star swings gives the planet's mass. It says nothing about the planet's size. Get both for one planet and you have mass and volume, which gives density, which tells you whether you are looking at rock, ice or gas.
Check yourself
Which method gives you each piece of information?
- The planet's diameter
- The planet's mass
- A chance to read the atmosphere in starlight passing through it
- A detection that works even when the orbit is tilted to our line of sight
- The fraction of the star's light blocked
- The star's speed toward and away from us
Show the answer
Transit (the dip): The planet's diameter, A chance to read the atmosphere in starlight passing through it, The fraction of the star's light blocked
Wobble (the Doppler shift): The planet's mass, A detection that works even when the orbit is tilted to our line of sight, The star's speed toward and away from us
A USEFUL BUT NARROW IDEA
The habitable zone
The habitable zone is the band of distances from a star where a planet with the right atmosphere could hold liquid water on its surface. Too close and it boils away; too far and it freezes. A dim red dwarf's zone is very close in, a bright star's is far out. It is a first filter for deciding where to look, not a verdict on whether anywhere is habitable.
Its limits matter. Venus sits near the inner edge of the Sun's zone and has a surface hot enough to melt lead, because of its atmosphere. Europa and Enceladus, moons of Jupiter and Saturn, are far outside the zone and appear to hold liquid oceans under ice, warmed by tides rather than sunlight. The zone is about surface water, and neither guarantees nor is required for life.
Check yourself
A planet found inside its star's habitable zone can still have a surface hot enough to melt lead.
Show the answer
True
True, and the example is in our own system. The zone only says the distance is right for liquid surface water, given a suitable atmosphere. Venus sits at the edge of the Sun's zone, and its thick carbon dioxide atmosphere traps so much heat that its surface would melt lead. Distance is a first filter, not a verdict.
Check yourself
Azadeh finds a planet with two measurements: a 0.03% transit dip on a Sun-sized star every 300 days, and a wobble giving it about four Earth masses. What can she honestly say?
- It is inhabited, since it is in the habitable zone
- About twice the Earth's width and four times its mass, so probably rocky
- It must be a gas giant, because four Earth masses is far too heavy for a rocky planet
- Nothing, because the planet has not been seen directly
Show the answer
About twice the Earth's width and four times its mass, so probably rocky
Right. The square root of 0.0003 is about 0.017, so the planet is roughly 1.7% of the star's width, near twice the Earth's. Four Earth masses in that volume gives a density like rock. A 300-day year around a Sun-like star puts it in the right distance band, which is a reason to look closer, not a conclusion about life.
What you can now do
- Read a claim for its method. Every fact in this course came from a measurement: a parallax shift, a spectral line, a dip in brightness, a stretched wavelength, a bent image, a ripple in spacetime.
- Use mass as the first question. It decided how a star lives and dies, whether a remnant is a white dwarf, a neutron star or a black hole, and what a planet is made of.
- Keep distance and time joined. Looking far is looking back, from 8 minutes for the Sun to 13.8 billion years for the microwave background.
- Separate the measured from the named. The 95% of the universe we call dark is measured; the names are placeholders. Saying so clearly is the difference between science and confidence.
Check yourself
One last pass over the whole course: match each thing we know to how we know it
Show the answer
- The distance to a nearby star → Its tiny shift as the Earth crosses its orbit
- What a star is made of → The dark lines in its spectrum
- That the universe was once hot and dense → A microwave glow arriving from every direction
- That a galaxy holds invisible mass → Stars at its edge orbiting far too fast
- The width of a planet we cannot see → How deeply it dims its star as it crosses
Lesson and course recap
- Planets are found by the dip they cause when crossing their star, and by the wobble they cause in its motion. Transits give size, wobbles give mass, and together they give density.
- The habitable zone is the distance band where surface liquid water is possible. It is a place to look, not a promise, and moons far outside it may hold oceans under ice.
- Starlight filtered through a planet's atmosphere during a transit reveals what that atmosphere contains, using the same spectral lines you met in lesson 02.
- There is no confirmed evidence of life beyond Earth, and with one example we cannot estimate how common it is. That is the honest answer, and it is the one worth keeping.
- Everything in this course rests on a measurement someone made. When you next read a claim about the universe, the useful question is the one you have been practising: how do we know?